Since no one is bringing practical experience to this thread, I’ll mention the theory.
What you have to do is look at intent. Google “sapphire engagement ring” and see what kind of pages come up in SERP. In my location, all ten pages are category pages from a catalog. Not product pages, not blogs. It looks unlikely that any type of page other than a category page will be able to compete in this SERP.
However, in the People Also Ask section of the same SERP, I can see a number of results from blog posts about sapphire vs diamond rings, where issues of cost and appearance are addressed. So perhaps you could optimize your category page for your main keyword, and then write a blog post on sapphires vs diamonds. This will give you a chance to rank both pages on the same SERP – one in results and one in questions.
As to the product pages, I don’t think there is any sense optimizing them for keywords. Product pages usually rank for very specific queries with transactional intent. Like if you google “buy Lenovo X1 Carbon 7” it will probably show you a couple of product pages from local or national stores. But with rings people are not looking for any particular one, there are no product names, they look for a selection, so a category page is probably as far as you should go with keyword optimization.